The invention relates in general to pipelaying vessels and in particular to a semi-submersible pipelaying vessel.
Semi-submersible pipelaying vessels are well known. These vessels have a relatively low surface area exposed to the wave action. Hence, the semi-submersible structure provides excellent motion characteristics in the presence of wave action.
In the design of a semi-submersible vessel for a pipelaying operation, two basic configurations have been used. In one configuration, the pipeline path, (the path along which the pipeline lengths are processed to form the continuous pipeline which is payed out from the stern of the vessel), extends from the bow of the vessel, above the upper deck surface, to the stern of the vessel where it leaves the vessel and is thereafter supported by a trailing, downwardly inclined, ramp extension. In an alternate configuration, the pipeline path extends from the bow of the vessel, above the upper deck, to a stern position below the upper deck. The alternate configuration is implemented, according to the prior art, in one of two alternate embodiments. In the first of the alternate embodiments, the pipeline path extends longitudinally down the center of the barge, thereby dividing the upper deck work area into port and starboard halves. A ramp, cut into the deck section toward the stern of the vessel, supports the pipeline as the pipeline path extends below the deck surface. In the other alternate embodiment, according to the prior art, the pipeline path is placed outside of the upper deck support columns, so that, in effect, the upper deck surface of the vessel is extended beyond the supporting column elements. This embodiment provides a potential instability since a significant portion of the vessel weight is cantilevered outside the support columns. In addition, in each of the alternate embodiments, the traditional design found in standard pipelaying barges must be modified because of the required placement of the pipe path.
A principal object of the invention therefore is a semi-submersible pipelaying vessel wherein the pipeline path does not interrupt or divide the deck surface and wherein the pipeline path further does not require a ramp extension for pipeline support at the stern of the vessel. Other objects of the invention are a semi-submersible vessel which provides an efficient functional upper deck work area, which can advantageously use the pipelaying configuration of a conventional pipelaying barge, which provides protection against wave action for the pipeline as it exits from the semi-submersible deck area of the vessel near the surface of the water, and which provides cost-effective use of the vessel.